Sewage sludge treatment

ABSTRACT

An apparatus and method for treating organic waste sludge such as sewage sludge is disclosed wherein the sludge is first dewatered, moved to a day hopper for storage, and then successively passed through first and second reactors. As the sludge is passed through the first reactor, in a continuous fashion, the sludge and acid are thoroughly mixed and has the pH thereof substantially lowered due to the addition of acid in the first reactor. The sludge is then moved through the second reactor where the sludge is subjected to a base material to substantially raise the pH thereof. The treated sludge is then pumped from the second reactor to a pugmill and then to a dryer which dries the material. The dried material is then suitable for use as a fertilizer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of Petitioners' earlier applicationSer. No. 10/291,892 filed Nov. 11, 2002, which is a continuation-in-partapplication of Petitioners' earlier application Ser. No. 08/926,109filed Sep. 9, 1997, entitled SEWAGE SLUDGE TREATMENT, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and method for treating organicwaste sludge and, more particularly, to an apparatus and method fortreating sewage sludge.

2. Description of the Related Art

The efficient and commercially effective utilization of organic wastesludge such as sewage sludge is important for society, particularly withthe increase of population and demands which are made on land. Atpresent, sewage sludge is dried in large bed pans, and the dried productis used as a compost high in organics. Although such composts are usefulin enhancing the retention and storage of water in soil, the compostsare low in inorganics and, hence, have limited fertilizer properties.The drying process is environmentally undesirable in that offensiveodors are produced. Further, the prior art methods of drying the sludgesare capital-intensive.

Processes have been suggested for treating sewage sludge to sterilizeand disinfect the same. For example, South African Patent No. 89/6160discloses such a method which involves treating sewage sludge withanhydrous ammonia gas to increase the pH of the sludge to at least 10,followed by using sufficient inorganic acid to neutralize, orsubstantially neutralize, the sewage sludge/ammonia admixture. Theresulting product is a liquid which is said to be useful as afertilizer. It is further suggested that the liquid can be dried, e.g.,by evaporation.

A prior art process for treating organic material such as sewage sludgefor use as a fertilizer is disclosed in U.S. Pat. No. 5,393,317, and itis believed that the process described in the '317 patent is less thandesirable due to the time required for treating the same inasmuch as incertain steps thereof, the mixture is allowed to stand for at least 20minutes. Another prior art process for treating organic material such assewage sludge is disclosed in U.S. Pat. No. 4,743,287. It is alsobelieved that the '287 process is less than desirable, since water mustbe added to the mixture of organic material and major elements toproduce a moisture content of 12%-30% by weight. Still another processis disclosed in U.S. Pat. No. 5,443,613. It is also believed that theprocess of the '613 patent is less than desirable, since water must beadded to an acidified suspension. It is believed that those processesrequiring an addition of water to the product increase the amount ofdrying required and, hence, causes an increase in drying time.

SUMMARY OF THE INVENTION

According to the present invention, an apparatus and method is disclosedfor treating organic waste sludge such as sewage sludge with the sewagesludge being able to be used as a fertilizer. The sewage sludge which istreated by the method of this invention may be raw sewage sludge,activated sewage sludge, or aerobically or anaerobically digestedsludge. The sewage sludge is first dewatered by a mechanical dewateringapparatus of conventional design. The sewage sludge is mechanicallydewatered so that the sewage sludge is comprised of a predeterminedpercentage of dry solids. The dewatered sewage sludge is then moved intoa day hopper for storage until the sewage sludge is to be processed. Theday hopper has a live bottom therein so that the material therein may beconstantly circulated or agitated to prevent the same from coagulatingor caking at the bottom thereof. The dewatered sewage sludge or sludgecake is then delivered to a reactor pump which passes the sludge cake toa first non-pressurized acid reactor or mixer wherein the sludge cake ismixed with and treated with an acid to substantially lower the pHthereof. As the sludge cake is passed through the first acid reactor, ina continuous fashion, the sludge cake is subjected to a thorough mixingaction. The sludge cake is then passed, in a continuous fashion, fromthe first acid reactor by a screw conveyor or the like to a secondreactor wherein the sludge cake is mixed with and subjected to a basematerial to substantially raise the pH thereof. The treated sludge cakeis then pumped from the second reactor to a pugmill and then to a dryersuch as a rotary dryer or pulse combustion dryer.

The dried product from the dryer is supplied to a cyclone separator,with the finished product resembling a fine powder or granular materialdepending on the dryer being utilized. The treatment of the sludge cakewith an acid, then a base, and then drying the same results in abacteria-free, pathogen-free product which has very little, if any,offensive odor, with the finished product being suitable for use as afertilizer.

The invention disclosed in the co-pending application has been proven towork at a site in Payson, Ariz. In the apparatus of the co-pendingapplication, the acid reactor is a closed, pressurized reactor. It hasbeen discovered that the pressures in the acid reactor may result inleaks around the bearing areas. Further, since the acid reactor in theco-pending application is closed, it is impossible to observe the mixingaction within the acid reactor or mixer. In some cases, it has beenfound that the mixing action in the acid reactor of the co-pendingapplication is not as thorough as is desired. The instant applicationprovides at least a pair of elongated rotatable shafts having mixingpaddles mounted thereon which extend transversely from the shafts andwhich are either right-hand or left-hand paddles which ensure that theacid is properly mixed with the dewatered sludge passing through theacid reactor.

It is therefore a principal object of the invention to provide animproved apparatus and process for treating sewage sludge.

Still another object of the invention is to provide a process fortreating sewage sludge wherein the final product is substantiallybacteria-free, pathogen-free and which may be used as a fertilizer.

Still another object of the invention is to provide a continuousprocess, as opposed to a batch process, for treating sewage sludge.

Yet another object of the invention is to provide an improved acidreactor or mixer which thoroughly mixes acid with the dewatered sludge.

Still another object of the invention is to provide a non-pressurizedacid reactor or mixer having improved mixing capability.

Still another object of the invention is to provide a process fortreating sewage sludge wherein the sewage sludge is converted fromsludge cake to a finished product in a short period of time.

Still another object of the invention is to provide a process fortreating sewage sludge which substantially reduces, if not eliminates,offensive odors normally associated with sewage sludge processing.

Still another object of the invention is to provide a process fortreating sewage sludge which can be computer-controlled.

These and other objects will be apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the apparatus of this invention;

FIG. 2 is a schematic of the apparatus and process;

FIG. 3 is an enlarged schematic of a portion of FIG. 2;

FIG. 4 is an enlarged schematic of a portion of FIG. 2;

FIG. 5 is a partial schematic of the apparatus of this invention;

FIG. 6 is a top view of the improved acid reactor or mixer of thisinvention;

FIG. 7 is a partial end view of the mixer of FIG. 6;

FIG. 8 is a side elevational view of the mixer of FIG. 6; and

FIG. 9 is a top view of the mixer of FIG. 6 depicting its relationshipwith other components of the apparatus of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One apparatus which is used to practice the process of this invention isillustrated in the drawings in FIGS. 1-4 while FIGS. 5-9 illustrated animproved acid reactor and associated structure. Although the drawingsillustrate the preferred embodiment of the apparatus which is used forpracticing the preferred embodiment, certain changes may be made thereinwithout departing from the spirit of the invention.

Referring to FIGS. 1-4, the sewage sludge which is treated by the methodof this invention may be raw sewage sludge, activated sewage sludge, oraerobically or anaerobically digested sludge. The sewage sludge ismechanically dewatered through the use of a belt-press dewateringapparatus of conventional design. Preferably, the sewage sludge isdewatered to a solid content of 18%-22%, with a 20% solid content beingpreferred. The dewatered sewage sludge is commonly referred to as sludgecake, with the consistency of the same typically being coherent andnon-flowable and, due to the moisture content, it will be wet.

The dewatered sewage sludge is delivered to the upper end of a dayhopper 10 which has a conventional live bottom apparatus 11 at its lowerend to circulate the materials in the day hopper to prevent thecoagulation or caking of the same. Preferably, day hopper 10 includes alevel sensor 12 which will control the delivery of the sludge cakethereto. The numeral 14 refers to a sludge cake pump which is incommunication with the lower end of the day hopper 10 and which has adischarge line conduit 16 extending therefrom which includes a pressuresensor 18. Discharge line 16 is in fluid communication with a reactorpump 20 which pumps the sludge cake to the inlet 22 of a reactor 24.Reactor 24 includes a high shear paddle mixer assembly therein tosubject the sludge cake to a desirable high shear mixing action. Thedischarge end of reactor 24 is connected to the intake end of a secondreactor 26 by means of pipe 28. Reactor 26 also includes a high shearpaddle mixer assembly therein to subject the sludge cake to the desiredhigh shear mixing action.

The intake end of reactor 24 is operatively connected to a source ofinorganic acid material such as phosphoric acid, sulphuric acid ornitric acid. The nature of the acid will be selected according to thenature of the inorganic component desired in the final product. Acidstorage tank 30 is connected to a supply pump 32 by line 34 having astop valve 36 therein. Supply pump 32 is preferably connected to smalleracid tank 38 which in turn is connected to an electronic valve assembly40 through pump 42 and check valve 44. Valve 40 is connected to theintake end of the first reactor 24 by means of line 46.

The numeral 48 refers to a storage tank for the base material such asammonia, ammonium hydroxide, potassium hydroxide, sodium carbonate,sodium hydroxide, lime or magnesite. The preferred base is ammonia gas,preferably anhydrous ammonia gas. Tank 48 is connected to electricallyoperated valve assembly 50 by means of line 52 having stop valves 54 and56 imposed therein. As seen in the drawings, line 52 also has a checkvalve 58 imposed therein. Valve assembly 50 is connected to the intakeend of reactor 26 by means of line 59 having check valve 60 therein.

Line 62 extends from the discharge end of reactor 26 and includes apressure sensor 64 therein. Line 62 extends to a three-port, two-wayvalve 66 having lines 68 and 70 extending therefrom. Line 70 extends toa backwash tank 72 including a level sensor 74. Line 76 extends frombackwash tank 72 to line 16, where it is connected thereto at 78. Stopvalve 80 is preferably imposed in line 76, as indicated.

Line 68 extends to a dryer assembly referred to generally by thereference numeral 82, which is preferably of the pulse combustion typealthough a rotary dryer will also work in a satisfactory manner. Pulsecombustion dryers are described in U.S. Pat. Nos. 4,708,159; 4,819,873;4,838,784; 4,941,820; and 4,992,039. The pulse combustion dryer utilizedin this invention utilizes high sound levels to enhance the dryingprocess. In the pulse combustion dryer of this invention, the treatedsludge cake is subjected to a high temperature pulsing high sound levelairstream which atomizes the material. It is believed that pulsecombustion drying enhances the sterilization of the product due to thefact that any pathogens remaining in the product will be atomized anddestroyed.

The sludge cake preferably passes through the pulse combustion dryer inapproximately fifteen to twenty seconds with the temperature thereinbeing approximately 235°-265° F. Dryer 82 includes a pulse combustionburner 84 having a supply of fuel being delivered thereto by means ofline 86. Combustion air is supplied to the pulse combustion burner 84 bymeans of the combustion air fan 88 which is in communication with asource of air 90. Combustion air fan 88 has a conduit or the like 92extending therefrom which delivers combustion air to the pulsecombustion burner 84 by means of conduit 94 and which delivers diluentair to the upper end of the dryer chamber 96 by means of the conduit 98.Conduits 94 and 98 are provided with dampers 100 and 102, respectively.

The dried product is discharged from the dryer chamber 96 by means ofthe discharge conduit 104 and is preferably supplied to a cycloneseparator 106 by means of conduit 108. Air from the separator 106 isdelivered to a scrubber assembly 110 by means of air line 112. Thefinished product is collected at the lower end of the cyclone separatorwhich is preferably provided with a rotating valve 112. Washwater isprovided to the scrubber 110 by means of washwater line 116 being incommunication with line 118. Line 118 is also in communication with thebackwash tank 72, as indicated. The numeral 120 refers to a slurry linewhich extends from the lower end of the scrubber assembly 110.

Preferably, the finished product is taken from the cyclone separator 106and is delivered to a module referred to generally by the referencenumeral 121, where the finished product is compacted, screened, ground,weighed, etc. If desired, the module 121 may be connected to the dustsuction inlet of the scrubber 110 by means of conduit 122.

When it is desired to process or treat the sludge cake in the day hopper10, the pumps 14, 20, 32 and 42 are activated. At the same time, thepulse combustion dryer assembly is activated, as will be the cycloneseparator, scrubber, etc. Initially, as the dryer is heating to itsdesired temperature, the sludge cake will be passed through the reactors24 and 26 and will be recirculated through the backwash tank by means oflines 62, 70 and the line 76. After the dryer has sufficiently heated,line 70 will normally not be utilized. As previously stated, the sludgecake is subjected to high shear agitation or mixing in reactor 24 and inreactor 26. The acid is mixed with the sludge cake in reactor 24 so thatthe pH of the sludge cake is substantially lowered to approximately 0.5to 1.5, with the preferred pH being approximately 1.0. As the treatedsludge cake subsequently passes through the reactor 26, sufficient basematerial is mixed therewith to substantially raise the pH thereof. ThepH of the sludge cake is preferably raised to approximately 4.3 to 5.5in the second reactor 26, with the preferred pH being 4.5. The reactionof the base material with the sludge cake in reactor 26 will cause thetemperature of the same to be raised to approximately 85°-95° C.Preferably, it should take approximately 1.5 minutes for the sludge caketo pass through reactor 24 and will take approximately 1.5 minutes topass through reactor 26. The retention time in the reactors will dependupon the volume of material being treated and the particular acids andbases being used.

As stated, the inorganic acid will typically be phosphoric acid,sulphuric acid or nitric acid. The nature of the acid will be selectedaccording to the nature of the inorganic component desired in the finalproduct. The amount of inorganic acid used will also depend on thequantity of inorganic components required in the powdered or granularfinal product. Heat is generated from the contact between the acid andthe sludge cake, although further heat may be added if required. Heathas the effect of killing the pathogens in the sludge cake. Acidtreatment has the effect of hydrolyzing organic material in the sludgeand reducing the viscosity of the sludge cake, making it flowable. Asalso stated, the base which is used and the concentration thereof willbe determined by the inorganic component required in the final product.

A very important part of the invention of FIGS. 1-4 is the use of thepulse combustion dryer. In such a dryer, sound pressures and heat aregenerated in a combustion chamber and are used to dry the product. Thesound pressures and heat are generally passed into the drying chamberand the treated sludge is then introduced into the path of the soundpressures and heat in the drying chamber. Sound pressures as high as 180dBA and combustion temperatures in the amount of 235°-265° F. areproduced in the dryer. The sound pressures disperse the sludge or slurryinto droplets which are dried by the heated air. A fine powder isproduced, as previously described. When fine powders are not desired, aconventional rotary dryer is the preferred drying apparatus.

FIGS. 5-9 illustrate the improved acid reactor and related mechanisms orstructures of this invention which ensure that the dewatered sludge willbe properly mixed with the acid while passing through the acid reactor.As seen in FIG. 5, the dewatered sludge is transferred from the mainhopper 200, through line 202 to line 204 which is in communication withthe acid being discharged from an acid pump.

The improved acid mixer or reactor of this invention is designated bythe reference numeral 206. Acid reactor 206 is comprised of an elongatedbody or housing 208 having opposite ends 210 and 212. Body 208 is anon-pressurized body having an open upper end 209 which is selectivelycovered with an anti-splash cover. An electric gear motor 214 or thelike is mounted on end 210 of body 208 and has a rotatable drive shaft216 extending therefrom which drives gears 218 and 220 which drivesquare shafts 222 and 224 in the same direction, respectively. Shafts222 and 224 are suitably rotatably mounted in body 208 (FIG. 7).Although a pair of shafts 222 and 224 are illustrated, two or moreshafts may be utilized in the acid reactor 206. One end of shaft 222preferably has a screw conveyor 226 mounted thereon which extends fromend 212 through a tube 228 having a discharge pipe 230 extendingtherefore. Pipe 230 is suitably connected to the intake side of aperistaltic metering pump 232 by a pipe or conduit 234.

A plurality of spaced-apart paddles 236 are preferably mounted on shaft222 along the length thereof while a plurality of spaced-apart paddles238 are preferably mounted on shaft 224 along the length thereof. Anydesired number of paddles and the spacing thereof may be utilized. Asseen in FIG. 6, the paddles 236 are longitudinally offset with respectto paddles 238 to prevent interference therebetween while providingenhanced mixing action. It is preferred that the paddles 236 on shaft222 be left-handed while the paddles 238 on shaft 224 be right-handed.The paddles may be interchanged between shafts left-hand for right-handto create intermittent reverse flow and obtain optimum mixing. Each ofthe paddles 236 comprises a square hub 240 which receives shaft 222 anda paddle arm 242 extending transversely therefrom. Each of the paddles238 comprises a square hub 244 which receives shaft 224 and a paddle arm246 extending transversely therefrom.

The sludge is introduced into one end of body 208 by means of a feedpipe 248 provided on line 204. An acid injector 250 extends into thefeed pipe 248 and is in communication with a source of acid. As thesludge and acid passes through the reactor 208 in a non-pressurizedstate, the mixing paddles 236 and 238 thoroughly mix the acid andsludge. The intermittent reverse flow described above ensures optimummixing. The fact that the reactor 206 is not pressurized eliminates theproblem of leaking bearings or the like. Further, the fact that the body208 has an open top covered with a removable cover or splash shieldenables the operator to view the mixing action within the body.

As seen in FIG. 5, the acidized sludge flows from reactor 206 to thepump 232 which then pumps the acidized sludge to the second reactor 242wherein ammonia is metered thereinto by a suitable metering device. Thetreated sludge is delivered to a pugmill 254 and then to a rotary dryerfor drying. After drying, the product is handled as describedhereinbefore.

Thus it can be seen that the invention accomplishes at least all of itsstated objectives.

1. A reactor for use in treating organic waste sludge, comprising: anelongated, generally horizontally disposed reactor body having oppositeends and a selectively closable open upper end; at least first andsecond horizontally spaced-apart shafts rotatably mounted in saidreactor body; each of said shafts having a plurality of spaced-apartmixing paddles mounted thereon which extend therefrom; an inlet means incommunication with said body for introducing organic waste sludge andeither an acid or a base material thereinto; an outlet means adjacentone end of said body permitting the mixture to be discharged from saidbody.
 2. The reactor of claim 1 wherein right-hand paddles are mountedon said first shaft and left-hand paddles are mounted on said secondshaft.
 3. The reactor of claim 1 wherein right-hand and left-handpaddles are mounted on said first and second shafts.
 4. An apparatus fordisinfecting organic waste sludge so that it may be used as afertilizer, comprising: means for dewatering sludge; a first acidreactor including an elongated, non-pressurized mixing body havingopposite ends and having at least two rotatable shafts therein whichhave mixing paddles mounted thereon for rotation therewith; means forsupplying the dewatered sludge to said first acid reactor adjacent oneend thereof; means for introducing acid to said mixing body of saidfirst acid reactor adjacent one end thereof; means for rotating saidshafts so that the dewatered sludge will be mixed with the acid wherebythe pH of the dewatered sludge will be reduced to a level to killsubstantially all the bacteria therein; a second base material reactor;means for delivering the acidized sludge from said first acid reactor tosaid second base material reactor so that the acidized sludge is passedthrough said second reactor; means for drying the treated sludge afterpassing through said second reactor.
 5. The apparatus of claim 4 whereinthe paddles on said shafts are right-handed and left-handed and mayinterchanged with one another to produce intermittent reverse flow ofthe dewatered sludge in said mixing body of said first acid reactor.